FIG. 1 of the accompanying drawings schematically illustrate the configuration of a known projection type image display apparatus. The image display apparatus comprises a reflector plate 100 having a reflecting surface of paraboloid of revolution, a light source 101 arranged at the focal point of the reflector plate 100, an integrator 102, a red color separating mirror 103R, a green color separating mirror 103G and a blue color separating mirror 103B.
The image display apparatus additionally comprises a cuboidal color synthesizer prism 104 so arranged as to have one of its surface 104G be stricken by green color light reflected by the green color separating mirror 103G, a reflector mirror 105R so arranged as to cause red color light reflected by the red color separating mirror 103R to strike another surface 104R of the color synthesizer prism 104 perpendicular to the surface 104G, another reflector mirror 105B so arranged as to cause blue color light reflected by the blue color separating mirror 103B to strike still another surface 104B of the color synthesizer prism 104 parallel to the surface 104R, an optical path length regulating lens 106 arranged between the green color separating mirror 103G and the blue color separating mirror 103B and another optical path length regulating lens 107 arranged between the blue color separating mirror 103B and the reflector mirror 105B.
The image display apparatus comprises still additionally a light valve for red light 108R arranged between the reflector mirror 105R and the surface 104R of the color synthesizer prism, a lens 109R arranged between the reflector mirror 105R and the light valve for red light 108R, a light valve for green light 108G arranged between the green color separating mirror 103G and the surface 104G of the color synthesizer prism, another lens 109G arranged between the green color separating mirror 103G and the light valve for green light 108G, a light valve for blue light 108B arranged between the reflector mirror 105B and the surface 104B of the color synthesizer prism and still another lens 109B arranged between the reflector mirror 105B and the light valve for blue 108B.
The image display apparatus further comprises a projector lens 110 arranged opposite to the surface of the color synthesizer prism 104 parallel to the surface 104G.
In a projection type image display apparatus having the above described configuration, a white lamp such as a xenon lamp or a metal halide lamp is typically used for the light source 101. Light emitted from the light source 101 is reflected by the reflector plate 100 and both ultraviolet rays and infrared rays are removed from reflected light by means of a cut filter (not shown) before reflected light is made to strike the red color separating mirror 103R. Of the light striking the red color separating mirror 103R, red color light is reflected by the red color separating mirror 103R and further by the reflector mirror 105R before it passes through the lens 109R and the light valve for red light 108R and enters the color synthesizer prism 104. Light other than red Of the light striking the green color separating mirror 103G, green color light is reflected by the green color separating mirror 103G and then enters the color synthesizer prism 104 by way of the lens 109G and the light valve for green 108G. Of the light striking the green color separating mirror 103G, light other than green color light passes through the green color separating mirror 103G and strikes the blue color separating mirror 103B by way of the optical path length regulating lens 106.
Of the light striking the blue color separating mirror 103B, blue color light is reflected by the blue color separating mirror 103B and, after passing through the optical path length regulating lens 107, it is further reflected by the reflector mirror 105B and enters the color synthesizer prism 104 by way of the lens 109B and the light valve for blue light 108B.
All lights of different colors that enter the color synthesizer prism 104 are synthetically combined together by the color synthesizer prism 104 and passes through the projector lens 110 before projected onto a transmission type or reflection type screen.
Known projection type image display apparatus having the above described configuration is accompanied by the problem of a large spectrum width of light of each of the three primary colors separated by the respective color separating mirrors and a poor color purity. They are also accompanied by the problem of the difficulty with which the brightness of the entire quantity of light is regulated because a white lamp is used as light source and energized to show a predetermined level of brightness. Still additionally, they are accompanied by the problem of the extreme difficulty with which the brightness of each primary color is regulated because light of each primary color is separated by means of a color separating mirror.
The flux of light coming out of a white lamp normally shows a circular cross section. On the other hand, light valves to be irradiated with light normally have a rectangularly parallelepipedic profile. Therefore, the flux of light striking a light valve is required to have a diameter greater than the diagonal of the light valve in order to evenly irradiate the light valve with light. Then, there arises a problem of a poor irradiation efficiency of light emitted from the light source of any conventional image display apparatus.
As pointed out above, conventional projection type image display apparatus comprising a white lamp as light source are accompanied by a number of problems. In an attempt to avoid these problems, there has been proposed a projection type image display apparatus employing light emitting diodes of the three primary colors that are independent from each other as light source ensuring an enhanced level of color purity in order to replace the white lamp. Such a projection type image display apparatus shows an improved color reproduction ability over a wide spectral range due to the light source with high color purity such as that of light emitting diodes.
FIG. 2 shows an XY chromatogram illustrating the color reproduction spectrum of a projection type image display apparatus using light emitting diodes as light source, that of an CRT (Cathode-Ray Tube) using a fluorescent body and that of an NTSC (National Television System Committee) type image display apparatus, which are indicated respectively by symbols A1, A2 and A3 in FIG. 2.
As seen from FIG. 2, while the color reproduction spectrum of a conventional projection type image display apparatus using a white lamp as light source is narrower than that of a CRT using a fluorescent body, a projection type image display apparatus using light emitting diodes as light source with high color purity can provide a color reproduction spectrum wider than that of a CRT using a fluorescent body.
However, light emitting diodes provide a surface light source and rays of light emitted from the surface light source include extra-axial rays that are located off the optical axis of the optical system comprising the light emitting diodes. Then, rays of light including extra-axial rays emitted from the light source cannot evenly and uniformly irradiate a light valve and typically show a poor light irradiation efficiency.